Been working with PageFrog Instant Articles for some time now. See an older article on AMP en Instant Articles about that here. I had AMP working just fine, but Facebook Instant Articles would not work properly somehow. This despite the fact that I did add the RSS feed as instructed.

Claim Your URL

It seems you first also need to claim your url before you can add the RSS feed or API and for the you need to add the Facebook meta property in your header. PageFrog does not seem to have the option to just enter your page URL based on information from Facebook. It actually did not mention the need for this. But Facebook states you do and that otherwise you can not work with feeds either.

This means that you will have to enter these details yourself and make these load in the header. In the end I added the meta property using an add_action call inside the functions.php of my child theme(see WordPress Support thread here  as well):

add_action('wp_head', 'facebook_instant_articles');

function facebook_instant_articles() {

echo '<meta property="fb:pages" content="id-no" />' . "\n";
}

And that seems to work well. I was able to claim my url and then again saved my RSS feed provided by PageFrog.

Instant Articles Review

Now I just need to see if I will have to add another 10 blog posts that Facebook will register to have it all approved or whether it will start working out of the box now. Facebook  mentions the following in the steps to get publishing of FB Instant Articles going:

The Instant Articles team will review a sample batch of your Instant Articles before you can begin to publish. Make sure you format your articles properly and avoid common errors before submission. Learn more about the article review process.
Create 10 Articles:
You must create at least 10 articles in your Production library before submitting for review.
Submit for Review:
Our team will review your articles and provide feedback within 3-5 business days.
Resubmit, If Needed:
Resolve any feedback on design or policy violations.

And I saw that when I checked the production articles page on Facebook that ten articles are unpublished and waiting to be reviewed:

Once they are accepted the Facebook Instant Articles Feed should work like a charm.